The electrostatic spray coating device according to the invention is suited especially for electrostatic hand spray pistol with which electrically conductive coating fluid, especially so-called water-varnish (Wasserlack) is spray onto objects from a varnish pressure vessel.
From the state of the art determined by the German Patent Office:
______________________________________ SU 16 55 295 A3 US 39 71 337 DE 36 09 509 A1 DE 39 32 623 A1 SU 2 85 086 US 39 29 286 DE-OS 21 28 455 DE 41 06 074 A1 ______________________________________
A varnish pressure vessel of metal is known from DE-PS 34 40 38I C2. The known varnish pressure vessel is electrically conductive through its metal and must, therefore, be placed on an insulating table. The coating fluid is atomized by the spraying device, for example a hand spray pistol, and electro-statically charged with a voltage between 10 kV and 140 kV. There electromagnetic field lines are formed between the spraying device and the object to be coated, along which the powder particles fly onto the object. Since the coating fluid, for example water-varnish, is electrically conductive, over the coating fluid also the varnish pressure vessel is charged with high voltage. So that the varnish pressure vessel charged with high voltage cannot be contacted, either a fence of metal must be build around the varnish pressure vessel at a distance of approximately 2 m um (sic) of the varnish pressure vessel, or the varnish pressure vessel must be set up in a separate space. In both cases, either in the fence or to the separate space doors must be provided with safety switches which on opening of the doors interrupt the current feed to a high-voltage generator for the generation of high voltage and ground the whole high-voltage system over a separating grounding switch. A coating cabin provided with such safety switches or grounding switches is known from DE-PS 35 26 013 C 1. From DE-PS 29 37 890 C3 there is known a device for the feed of an electrically conducting varnish with an intermediate storage unit of electrically insulating material, into which the electrically conducting coating fluid is fed for the interruption of the electric voltage path. The intermediate storage unit consists of polyethylene and has an inset of metal which by the high voltage of a spraying device atomizes and electrostatically charges, is likewise charged through the coating material which is fed over a tube to the spraying device. This state of the art shows that the "kick-back" ("Zuruckschlagen") of the high voltage of the spraying arrangement from the spraying device back through the electrically conducting coating fluid into the coating fluid feed system is a great problem, which the specialist had already attempted to solve in many ways.
In the known electrostatic spray devices for electrically conducting varnishes the varnish pressure vessel, the spray pistol and the tube through which the varnish flows from the varnish pressure vessel to the spray pistol form an electric capacitance. Especially when around the metal varnish pressure vessel the necessary perpetration protection (Begehungsschutz) is built of grounded metal grid fencing (Metallgitterzeun), we are dealing, through the dielectric, with air with a strong capacitor action, which drastically increases the total capacitance of the system. This capacitance can generate on the spray nozzle of the spray pistol a strong electrical discharge from the spray nozzle to an operating person. It is a matter here of the discharge that an operating person can draw upon himself if he holds his or her hand on the spray nozzle during the varnishing process. There the maximally admissible 350 mJ for personnel protection can be exceeded.